Fuel cells are classified into several types according to the electrolytes or electrodes used therein. Typical types are alkaline types, phosphoric acid types, molten carbonate types, solid electrolyte types and polymer electrolyte types. In particular, polymer electrolyte fuel cells that can operate at temperatures ranging from low temperatures (about −40° C.) to about 120° C. attract attention and are progressively developed and practically used as power sources for low pollution automobiles. The polymer electrolyte fuel cells are expected to be used as automobile drive sources or stationary power sources. The use in these applications requires long-term durability.
The polymer electrolyte fuel cell has a solid polymer electrolyte sandwiched between an anode and a cathode. A fuel is fed to the anode, and oxygen or air is supplied to the cathode, whereby oxygen is reduced at the cathode to produce electricity. The fuel is usually hydrogen or methanol.
To increase the reaction rate in fuel cells and enhance the energy conversion efficiency, a layer containing a catalyst (hereinafter, also the fuel cell catalyst layer) is conventionally provided on the surface of a cathode (an air electrode) or an anode (a fuel electrode) of fuel cells.
Here, noble metals are generally used as the catalysts. Of the noble metals, platinum that is stable at high potential and has high catalytic activity is most frequently used. However, since platinum is expensive and exists in a limited amount, alternative catalysts have been desired.
Further, the noble metals used on a cathode surface are often dissolved in an acidic atmosphere and are not suited in applications requiring long-term durability. Accordingly, it has been strongly demanded that catalysts are developed which are not corroded in an acidic atmosphere and have excellent durability and high oxygen reducing ability.
Materials containing nonmetals such as carbon, nitrogen and boron capture attention as alternative catalysts to platinum. The materials containing these nonmetals are inexpensive compared to noble metals such as platinum and are abundant.
Nonpatent Document 1 reports that zirconium-based ZrOxN compounds show oxygen reducing ability. Patent Document 1 discloses, as platinum-alternative materials, oxygen-reducing electrode materials containing a nitride of one or more elements selected from Groups 4, 5 and 14 in the long periodic table. Patent Document 2 discloses that a partial oxide of a compound of any of titanium, lanthanum, tantalum, niobium and zirconium and any of nitrogen, boron, carbon and sulfur is used as a fuel cell electrode catalyst. Patent Document 3 discloses that titanium carbonitride powder is used as an oxygen electrode catalyst for polymer electrolyte fuel cells.
However, the materials containing these nonmetals do not provide sufficient oxygen reducing ability for practical use as catalysts and their activity is insufficient for the materials to be practically used in fuel cells.
Patent Document 4 discloses an oxycarbonitride obtained by mixing a carbide, an oxide and a nitride and heat treating the mixture in vacuum or an inert or non-oxidative atmosphere at 500 to 1500° C.
However, the oxycarbonitride disclosed in Patent Document 4 is a thin-film magnetic head ceramic substrate material, and the use of the oxycarbonitride as catalyst is not considered therein.
Meanwhile, platinum is useful not only as a fuel cell catalyst as described above but as a catalyst in exhaust gas treatment or organic synthesis. However, the expensiveness and the limited amount of platinum have created a need of alternative catalysts in these applications too.
Patent Document 1: JP-A-2007-31781
Patent Document 2: JP-A-2006-198570
Patent Document 3: JP-A-2007-257888
Patent Document 4: JP-A-2003-342058
Nonpatent Document 1: Journal of The Electrochemical Society, S. Doi, A. Ishihara, S. Mitsushima, N. Kamiya, and K. Ota, 154 (3) B362-B369 (2007)